I've 5 avocado trees in my garden, I attach you a couple of pics.
According to my experience I give you my one cent opinion.
1- Chose well your young trees before buying: don't pick the big ones of the bunch in small pots or you will end for sure with a root bonud tree. Small tree in a decent size pot is much better. don't worry for the size, once in the ground, if the conditions are right they will grow like a rocket.
This tree was put in the soil exactly one year ago. It was 2 feet high now it's 10 feet.
2- Among all the trees i have avocado are probably the most root-sensitive ones. when i transfer lemons i usually massage the rootball and often cut as much as 50% of roots to spread them. For avocad that's not allowed. when you transfer them into the soil, be careful not to damage the roots, try to touch or "disturb" them as less as possible, they are brittle and quite prickly
Prepare everything beforehand so that at the last moment you get the plant from the pot (if necessary cut it) and the tree will go straight into the ground. Then water abundantly but don't make the soil damp and muddy.
3- In my place avocado can still grow in awful limestone soil with a 7.5 pH where other trees would struggle just to survive.
BUT at one condition: a PERFECT DRAINAGE. No wet feet, no stagnant water. Wet feet = phytophtora root rot = poor growth or death this is a MUST for avocado.
4- Keep a wide area around your trees clear of weeds. DOn't just cut them but eradicate them completely. Remember the point 2? Roots are touchy and they don't like competition with hungry and invading weeds. This is especially true for young trees and in dry season when water is scarce.
As for my experience 90% of avocado health is below the ground. Helthy roots will make a healthy tree.
5- Keep your young trees in partial shade if the temperatures are high until they grow a bit and look strong. This is valid for my location just 10 °N from equator, sun is scorching for at least 6 hours a day.
6- Fertilize lightly but quite often and don't forget microelements
Last but not least: DO NOT GIVE UP!!!